At about 45 miles per hour the overdrive goes in and out once you get past that speed everything is okay.

It always starts out in first, it's a 3 speed automatic, that's what automatics do. they start out in first and at about 15-20 miles per hour , they shift to second gear, and when the speed is about 35-45 MPH, the transmission automatically shifts into third gear, or Drive.

Bigger automatics have 4 or more speeds. Till recently, 4 speeds were the norm, and fourth gear was often dubbed Overdrive, as it allowed the engine to run slower in 4th gear, than it had to maintain speed in 3rd gear, drive. It's good, it increases your mileage per gallon of gas, so you save that way, but like all things more complicated, they cost more to fix when something goes wrong.

OVERDRIVE is fine in most driving. if you tow a trailler or pulling long hills take out of overdrive. otherwise the vehicle will handle when to shift.

is this a three speed trans? or 4spd w/overdrive? either way i believe you have a pressure problem whether a torque converter or a trans pump. I was wondering how you know all gears worked if it will only do 30 mph as the 4th or overdrive usually wont engage at that speed. but here say,

Check for a transmission speed sensor fault (OBDII scan). Loss of speed sensor signal will result in no torque converter lock up, and no overdrive. Also transmission filter should have been changed at 80,000 miles. Age of your vehicle(1994) warrants the filter service. Transfer case (4WD) needs to be serviced as well.

suspect cruse control setting.
some cruse control settings can be made so that no matter how hard you press the pedal it will only go to the set speed. check manual for any secondary cruise control setting that might have been set to 45.

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Yeah you may need a whole new dashboard because the whole panel circuit for may be messed up.

Ck your exhaust pipe at rear of car noting any obnoxious smell like rottrn eggs. Hold a piece of paper, I use a dollar bill ,in in front of the exhaust. If it blows the paper gently, you have a catalyctic converter going or is bad. Will need replace if so.

check the sparkplug wires on the firewall side for arching-body seam there that will cut them